The Ovonic EEPROM is a proprietary, high performance, non-volatile, thin-film electronic memory device. Its advantages include non-volatile storage of data, potential for high bit density and, consequently, low cost because of its small footprint and simple two-terminal device configuration, long reprogramming cycle life, low programming energies and high speed. The Ovonic EEPROM is capable of both analog and digital forms of information storage. Digital storage can be either binary (one bit per memory cell) or multi-state (multiple bits per cell).
The general concept of utilizing electrically writable and erasable phase change materials (i.e., materials which can be programmed between generally amorphous and generally crystalline states) for electronic memory applications is known in the art, as is disclosed, for example, in U.S. Pat. Nos. 3,271,591 and 3,530,441 to Ovshinsky both of which are assigned to the same assignee as the present invention, and both disclosures of which are incorporated herein by reference.
The early phase-change materials described in the '591 and '441 patents were based on changes in local structural order. The changes in structural order were typically accomodated by atomic migration of certain species within the material. Such atomic migration between the amorphous and crystalline states required a time necessary to accomodate the migration, thereby making the programming times and programming energies relatively high.
The relatively slow (by present standards) programming speed, particularly when programmed in the direction of greater local order (in the direction of increasing crystallization), and the relatively high input energy needed to initiate a change in local order where limitations which prevented the memory cells described in the '591 and '441 patents from widespread use as a direct and universal replacement for present computer memory applications, such as tape, floppy disks, magnetic or optical hard disk drives, solid state disk flash, DRAM, SRAM, and socket flash memory.
The most significant of these limitations was the relatively high energy input required to obtain detectable changes in the chemical and/or electronic bonding configurations of the chalcogenide material in order to initiate a detectable change in local order. The electrical energy required to switch these materials typically measured in the range of about a microjoule. It should be noted that this amount of energy must be delivered to each of the memory elements in the solid state matrix of rows and columns of memory cells. Such high energy levels translate into high current carrying requirements for the address lines and for the cell isolation/address device associated with each discrete memory element.
Low programming energy is especially important when the EEPROMs are used for large scale archival storage. Used in this manner, the EEPROMs would replace the mechanical hard drives (such as magnetic or optical hard drives) of present computer systems. One of the main reasons for this replacement of conventional mechanical hard drives with EEPROM “hard drives” would be to reduce the comparatively large power consumption of the mechanical systems. In the case of lap-top computers, this is of particular interest because the mechanical hard disk drive is one of the largest power consumers therein. Therefore, it would be especially advantageous to reduce this power load, thereby substantially increasing the operating time of the computer per charge of the power cells. However, if the EEPROM replacement for mechanical hard drives has high switching energy requirements (and therefore high power requirements), the power savings may be inconsequential or at best unsubstantial. Therefore, any EEPROM which is to be considered a universal memory requires low programming energy.
Also significant were the switching times of the electrical memory materials described in the Ovshinsky patents. These materials typically required times in the range of a few milliseconds for the set time (the time required to switch the material from the amorphous to the crystalline state); and approximately a microsecond for the reset time (the time required to switch the material from the crystalline back to the amorphous state).
Electrical phase change materials and memory cells having decreased electrical switching times and programming energies are described in commonly assigned U.S. Pat. No. 5,166,758 to Ovshinsky, the disclosure of which is incorporated by reference herein. Other examples of electrical phase change materials and memory cells are provided in commonly assigned U.S. Pat. Nos. 5,296,716, 5,414,271, 5,359,205, 5,341,328, 5,536,947, 5,534,712, 5,687,112, and 5,825,046 the disclosures of which are all incorporated by reference herein. Still further examples of electrical phase change materials and memory cells are provided in commonly assigned U.S. patent application Ser. Nos. 08/878,870, 09/102,887, and 08/942,000 all of which are incorporated by reference herein.
Generally, the disclosed phase change materials can be electrically switched between structural states of generally amorphous and generally crystalline local order. The materials may also be electrically switched between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. That is, the switching of such materials is not required to take place between completely amorphous and completely crystalline states but rather can be switched in incremental steps reflecting either (1) changes of local order, or (2) changes in volume of two or more materials having different local order so as to provide a “gray scale” represented by a multiplicity of conditions of local order spanning the spectrum between the completely amorphous and the completely crystalline states. The phase change materials exhibit different electrical characteristics depending upon its state. For example, in its amorphous state the material exhibits a higher electrical resistivity than it does in its crystalline state.
The phase-change materials are truly non-volative and will maintain the integrity of the information stored by the memory cell without the need for period refresh signals. As well, the subject materials are preferably directly overwritable so that they can be set to directly to one of a plurality of resistance values without the need to be set to a specific starting or erased resistance value, regardless of the previous resistance value of the material in response to a selected electrical input signal. Furthermore, the phase change materials preferably have a large dynamic range which provide for gray scale storage of multiple bits of binary information in a single cell by mimicking the binary encoded information in analog form and thereby storing multiple bits of binary encoded information as a single resistance value in a single cell.